The size of a telescope is measured in several ways. The 6" is the aperture of the telescope and the F# is the focal ratio or the focal length over the aperture of the telescope.
Short focal lengths allow for wide field views while longer focal lengths allow for higher magnifications which are nice for looking at the planets.
As to making a telescope, there are a fairly large number of websites that describe "mirror making" and "telescope making" and a good googling will show up a large number of sites, including mine. I'll note that mirror making is quite easy but it can be more boring if you are a person that wants things now.